1. Field of the Invention
The present invention relates to a surface luminous lighting unit that is formed into a compact unit, and which is free of glares so as to effectively illuminate things to be illuminated.
2. Description of the Prior Art
The lamp structure of a conventional lighting unit will be described.
FIG. 6 shows the structure of a conventional lighting unit, such as disclosed in the official gazette of Japanese Patent Laid-Open No. 63-69102 (not examined), in which reference numeral 1 denotes a lamp, reference numeral 2 a light-transmitting light panel, reference numeral 3 a base of the lamp 1, and reference numeral 8 a lamp luminous portion that is the portion of the lamp 1 excluding the base 3 thereof. Reference X total length of lamp 1.
This lighting unit is intended to obtain illumination of an edge-light type by providing the light-transmitting light panel 2 and the lamp in parallel to each other. If the length of the lamp luminous portion 8 excluding the base 3 is represented as L1mp, the length of the longer side of the light panel 2 as Lpn1, the distance between the lamp 1 and the light panel 2 as A, and the angle of incidence as .theta. at which light enters the end portion of the light panel 2 from the end portion of the lamp luminous portion 8, the length of the lamp luminous portion 8 is expressed as EQU L1pm=2.times.A.times.tan .theta.+Lpn1,
where .theta. is in the range of 34.degree. to 50.degree..
It is possible to make uniform the luminance on the surface of the light panel by determining the length L1mp of the lamp luminous portion 8 and the length Lpn1 of the longer side of the light panel 2 in such a manner that the above equation is fulfilled.
Since the conventional lighting unit is constructed as described above, the range of the length Lpn1 of the longer side of the light panel is determined by the distance A between the lamp 1 and the light panel 2 and the length L1mp of the lamp luminous portion 8. For instance, if we now set A=3 mm, and L1mp=390 mm, the optimal length of the longer side of the light panel 2 falls within the range of 383 to 386 mm.
A graph shown in FIG. 7 shows the results of measuring the illuminance on the illuminated surface while altering the horizontal distance from the lighting unit wherein a lamp of total length.times.420 mm long (L1mp=390 mm) was provided at a position 3 mm away from the light panel (A=3 mm), the length of the longer side of the light panel 2 was changed so as to be 280, 310, 340, 370, and 400 mm, and the lighting unit was mounted at 300 mm high above the illuminated surface with the luminous surface of the light panel 2 being faced downwardly. It is understood from this graph that there is little difference in illuminance distribution until the length Lpn1 of the longer side of the light panel was reduced to 310 mm, while there was a difference when it was reduced to 280 mm. Therefore, when used as a part of a lighting unit, even if the length Lpn1 of the longer side of the light panel is set to be 310 mm, it is possible to obtain substantially the same degree of illuminance as would be obtained with the greater Lpn1s. Consequently, the rule expressed by the above equation is not appropriate.
FIG. 8 is a graph showing a relationship between the luminance and position of a fluorescent lamp 1, having a length of 420 mm and a diameter D off 16.7 mm, with filaments being disposed E=40 mm inwardly from the ends of the lamp, respectively. The luminance of the lamp 1 decreases towards the ends of the lamp, and falls drastically in the areas where the filaments 4 and bases 3 are provided. The difference in luminance causes an unevenness in the brightness of the light panel 2. Due to this, the length Lpn1 of the longer side of the light panel 2 should be set to be substantially the same as the distance F=340 mm between the filaments 4,4 of the lamp. Since the length Lpn1 of the light panel 2 is subject to substantially the same degree of illuminance with in the range from 310 to 400 mm, the length Lpn1 can be determined at 310 mm. Therefore, the rule stipulated by the above equation is not proper.
Thus, according to the above equation, there is a drawback in that the longer side of the light panel is made too long for use as a part of a lighting unit, eventually causing the light panel to become extremely heavy. Thus it is not appropriate to use the above equation to apply.
With the conventional lighting unit, there is also another drawback in that since it is not formed as a unit, a fixing means has to be provided for each place to be illuminated. Thus it is troublesome and consuming, because it takes much time to fix them. Additionally, it causes deteriorative operating efficiency.